In 1865 August Kekulé proposed a structure for benzene, C6H6: a ring of six carbon atoms joined by alternating single and double bonds, with one hydrogen atom on each carbon. In modern terms this would be cyclohexa-1,3,5-triene. Kekulé later suggested that the molecule switches rapidly between two equivalent structures in which the positions of the double bonds are swapped.
The model explained the formula C6H6 and allowed every carbon to form four bonds. However, it makes predictions that can be tested:• Bond lengths: with three C=C bonds and three C-C bonds, there should be two different bond lengths, about 0.134 nm for C=C and 0.154 nm for C-C. The ring would be an irregular hexagon.
• Reactions: with three C=C bonds, benzene should behave like an alkene, undergoing electrophilic addition easily and decolourising bromine water at room temperature.
• Enthalpy of hydrogenation: this should be three times that of cyclohexene, which has one C=C bond.None of these predictions matches real benzene (see the Evidence for Delocalisation lesson), so the Kekulé model was replaced by the delocalised model. Kekulé structures are still sometimes drawn, but they are not an accurate picture of the bonding.Worked example: cyclohexene has an enthalpy of hydrogenation of −120 kJ mol−1. The Kekulé structure has three C=C bonds, so its predicted enthalpy of hydrogenation is 3 × (−120) = −360 kJ mol−1.
The model explained the formula C6H6 and allowed every carbon to form four bonds. However, it makes predictions that can be tested:• Bond lengths: with three C=C bonds and three C-C bonds, there should be two different bond lengths, about 0.134 nm for C=C and 0.154 nm for C-C. The ring would be an irregular hexagon.• Reactions: with three C=C bonds, benzene should behave like an alkene, undergoing electrophilic addition easily and decolourising bromine water at room temperature.
• Enthalpy of hydrogenation: this should be three times that of cyclohexene, which has one C=C bond.None of these predictions matches real benzene (see the Evidence for Delocalisation lesson), so the Kekulé model was replaced by the delocalised model. Kekulé structures are still sometimes drawn, but they are not an accurate picture of the bonding.Worked example: cyclohexene has an enthalpy of hydrogenation of −120 kJ mol−1. The Kekulé structure has three C=C bonds, so its predicted enthalpy of hydrogenation is 3 × (−120) = −360 kJ mol−1.